Electronic piano



Sept. 13, 1960 c. w. AN DERSEN ELECTRONIC PIANO 2 Sheets-Sheet 1 Filed July 21, 1954 ll llll II Illllllll INVENTOR. 40 54% Zdfl/zdwzwm Sept. 13; 1960 c. w. ANDERSEN ELECTRONIC PIANO 2 Sheets-Sheet 2 Filed July 21, 1954 INVENTOR. 616/ fird 1% M76812 ELEQTRGNIC PIANO Clifford W. Andersen, De Kalb, 111., assignor to The Rudolph Wurlitzer Company, North Tonawanda, N.Y., a corporation of ()hio Filed July 21, 1954, Ser. No. 444,809

4 Claims. (Cl. 8 4-124) This invention is concerned generally with musical instruments, and more particularly with a portable electronic piano.

Pianos and related instruments for a great number of years have utilized tensioned strings as sound generators. In the case of pianos the strings have been actuated percussively by striking them with hammers whereas in other instruments the strings have been plucked.

In order to obtain bass notes with tensioned strings in a piano, the strings must be quite long and heavy. This requires a substantial length in the direction of the strings running from a minimum on the order of four feet for the smallest pianos to a maximum on the order of feet or more in grand pianos. In addition to this, the large number of strings and the tension on each one results in a total string tension or force on the order of several tons. Accordingly, strong and massive frames have been necessary to resist this force. Consequently, the smallest pianos have weighed several hundred pounds while the grand pianos have been on the order of of a ton or more.

The large size and weight of conventional pianos has rendered them more-or-less stationary in nature. It is true that they can be rolled on casters from one location to another on the same floor, but to move a piano from one floor level to another has been practical only in extreme cases. Furthermore, in home use the large size and weight of pianos has generally restricted them to use on the first floor and in the largest room, generally the parlor. Thus, when a student has desired to practice the piano, the remainder of the family has been annoyed, or has had to leave the parlor.

It has been proposed to produce an electronic piano for reducing the size and Weight heretofore necessary in piano construction. In accordance with such proposals hammers are caused to strike vibratory metallic reeds which are mounted in capacitive relation with suitable pick-up means. The capacity between each reed and associated pick-up means varies in accordance with the vibration of the reed, and an oscillating potential corresponding to the musical tone of the reed is developed. Such potential when suitably electronically amplified and transduced produces a piano-like tone. However, in prior instruments true piano tones have not been attained due to the inherent physical characteristics of the reeds and pick-up means, and due to the electrical characteristics of the amplifiers.

It is an object of this invention to provide an electronic piano having tones substantially indistinguishable from those of a conventional piano and much more lifelike than any heretofore known.

It is an object of this invention to provide an electronic piano wherein both electrical and mechanical characteristics of amplifying and transducing means are utilized to produce true piano-like tones.

More specifically, it an object of this invention to provide an electronic piano having a broad band amplifier for properly amplifying the percussive notes of a piano are a 2 and for properly amplifying all of the partials, combined with suitable electrical and mechanical components to limit the frequency output for producing realistic piano tones.

It is another object of this invention to utilize a loudspeaker in a specific arrangement for control of tonal effects in an electronic piano.

Still other objects of this invention include the provision of constructional features and arrangement of parts in an electronic piano to achieve small size and weight, and hence ready portability. A

Other and further objects and advantages of the present invention will be apparent from the following description when taken in connection with the accompanying drawings wherein:

Fig. 1 is a perspective view of a piano constructed in accordance with the principles of my invention;

Fig. 2 is a cross-sectional view taken substantially along the line 2-2 in Fig. 1;

Fig. 3 is a horizontal view partially in section taken substantially along the line 3--3 in Fig. 2;

Fig. 4 is a cross-sectional view showing the speaker mounting and taken substantially along the line 4-4 in Fig. 1;

Fig. 5 is a schematic wiring diagram of the amplifier in my piano; and

Fig. 6 is a cross-sectional view somewhat similar to Fig. 4 and showing a modified form of transducer.

Referring first to Figs. 1, 2, and 3 for a general description of the invention, there will be seen an electronic piano generally designated by the numeral 10. The piano includes a case 12 which may be supported on a surface such as a desk or a table, or which may be supported by steel or wrought iron legs 14 of the type currently popular for interior decoration.

The case 12 preferably is made of wood or of plywood and preferably is covered with a plastic or leatherette material. The case comprises a flat bottom 16, arear wall 18 upstanding therefrom, and'a pair-of end walls 20 upstanding from the rear walls. The end walls are provided with elongated horizontal slots 22 covered on the inside with grille cloth. These slots serve as finger grips for lifting the piano, and also serve as ports for controlling the back loading of the speaker as hereinafter will be brought out more fully. A suitable connector 26 is provided in one of the end walls 20 for cooperation with an electrical connector on a flexible power line 28 for supplying power to the piano.

The piano case also includes a top 30 parallel to the bottom 16 and terminated short thereof as at 32. A cover or lid 34- is hinged to the edge 32 of the top 30 as at 36, and extends diagonally downward to a depending edge portion 38 cooperable with an upstanding flange 40 on the bottom 16. Toggle fasteners 42 are provided on the edge portion 38 and the flange 40 to hold the cover in closed position. The cover is operable to the dashed line position shown in Fig. 2 and in such position cooperates with a tray 44 therebeneath to form a music rack, the tray having an upturned outer edge 46 to prevent sheet music from sliding from the tray.

A keyboard 48 is disposed beneath the cover or lid 34 with the latter in closed position, and is exposed for playing when the lid is raised. The keyboard comprises a plurality of keys 50 including both the white and black keys of a conventional piano keyboard. The number of keys is determined in accordance with any conventional piano keyboard, and in the illustrative embodiment of the invention there are sixty-one keys. The keys are arranged to pivot the hammers 52 of a plurality of corresponding piano actions 54. Each hammer is positioned to strike a vibratory metallic reed 56, there being one reed for each key of the keyboard, and the reeds varying in size and weight in accordance with their desired frequencies. The ends of some of the reeds may be provided with weights as indicated at 58 in order toachieve the desired vibratory characteristics and maid in turning the .reeds. bysimply adding to or removing fromtthe weights. Each reed-:is arranged to vibrateina slot- 60. in a metal b-ar62 forming a pick-up element extending across the piano. and insulatedly supported from a frame, the frame being supported onrubber blocks 64 on the base plate or bedj66 lying on top of the bottom 16. vA grounded sheet metal shield 67 overlies the pick-ups to prevent electrical interference. The reeds 56 normally are engaged by dampers (not shown) which are .retracted individually when the keys are depressed or which all may be retracted simultaneously by a suitable mechanism actuated by a footpedal 68 (Fig. 1) .detachably connected by means such as a Bowden cable 70 to the remainder of the piano.

An amplifier 72, the electrical construction of which will be discussed shortly, is supported from theunder side of the top 30 of the case 12 adjacent one side thereof and adjacent the forward or terminating edge 32 as Will be apparent from the drawings. A loudspeaker74 likewise is supported from the top 30 adjacent the front edge 32, and toward the opposite side from the amplifier. The speaker is of conventional construction including a driver unit 76 containing a voice coil and magnet, a speaker cone 78 and a frame 80. The speaker conveniently is of the'elliptical type commonly used in automobile radios and is of rather poor quality, particularly being deficient in bassresponse whereby partially to eliminate bass tones in order properly to simulate piano tones.

An opening 82 (Fig. 4) is provided in the top- 36 of the case and the speaker 74 is mounted below this opening and an alignment therewith, spacers 84 being provided to space the speaker at predetermined distance from the top 30 of the case, thereby further controlling the bass response of the instrument. A speaker cover 86 having grille cloth 88 on the under surface thereof is secured over the opening 82 by any suitable means such as screws 90,,and the speaker cover 86 is provided with elongated, rather narrow slots 92 acting with the space betweenthe speaker and the bottom of the top 30' to control the front loading of the speaker cone 78 to limit the bass response to a predetermined degree. Back loading of the speaker cone is determined at least in part by the slots 22 in the ends of the piano case as has been noted previously.

It further is contemplated that car phones might be utilized for listening to the piano, and for this purpose two: jacks 94-and 96 are provided adjacent the right end of the piano keyboard. The first jack 94 is arranged in circuit with the amplifier and loudspeaker in such a manner that when an ear phone plug is inserted, the speaker will automatically be switched out of the circuit and the sound will be reproduced only by the ear phones. The second jack 96 is arranged for connection with the ear phones in parallel with the speaker so that the piano tones can be heard simultaneously from the speaker and from ear phones plugged into the jack 96.

It is further contemplated that the speaker 74 in the conventional sense could be eliminated and the piano case itself could be used as a sound radiator. As shown in Fig. 6, top 30a of the piano case is imperforate. The loudspeaker is replaced by a sound unit 7412 including a driver unit 76a having a magnet'and voice coil. The voice coil is arranged to vibrate a rod 98 secured by a suitable fastener 100 to the under side of the case top 30a. The driver unit 76a may be supported from beneath by means of suitable bracket 102. When the voice coil vibrates the rod 98, the vibrations are transmitted to the case top 30a which then acts as asounding board or tone propagator to render the piano tones audible.

The physical parts of the piano now have been de-' scribed and it will be evident that the piano is small and compact. 'Excluding-the-legs, the pianois substantially 4 less than a foot in height, less than two feet in depth, and about three feet across in a specific illustrative embodiment of the invention. The weight of the illustrative embodiment is on the order of seventy to eighty pounds, and it thus will be seen that the piano is readily portable by means of ahandle 104 (Fig. 1) on the up- .turned front flange 40 of the case.

The amplifier is shown in the schematic diagram in Fig. 5 and includes a power supply indicated generally by the numeral 10-6. Thepower supply includes a transformer 108 having a primary winding 110 connected through a fuse 1.12 and a switch 114' to an electrical connector 116 cooperab le with the connector 26 on the line cord 28. A pilot light 118 is connected across the primary 110 and is disposed above the keyboard of the piano (not shown) for providing a visual indication as to whether the piano is on or off, the switch 114 being positioned next to the pilot light.

The transformer is provided with a high voltage sec-" ondary 120 center tapped at 122 and having the ends connected to the plates 124 of a full wave rectifier tube 125 such as a 5V 4G. The transformer also hasalow voltage filament winding 126 connected to the filament 128 and likewise to the cathode 130 of the rectifier. A power output connection 132 is provided from the cathode of the rectifier tube.

The transformer further includes a low voltage filament Winding 134 center tapped at 136 and connected to the filaments 138,140, 142, and 144 of the tubes shortly to be described. The power output connection 132 is connected to .a suitable filter 146 to provide filtered voltage to a B-lbus line 148.

The amplifier further is provided with a grounded input connection 151), and an ungrounded input connectron 152. The grounded input connection is connected in parallel to all of the vibratory reeds 56, While the urn grounded connection is connected in parallel to the metal bar 62 comprising the pick-up means. The ungrounded connection 152 is connected through a resistor 154' and through suitable decoupling networks comprising resistors 156, 158, and 160, and grounded capacitors 162, 164, and 166 to the B+ bus line 148', thereby applying potential to the pick-up means. The reeds and pick-up means act as an electrical capacitor, and this capacity varies when any one or more reeds vibrate in accordance with the vibrations. This change in capacity with the direct current potential applied to the capacitors through the load res1stor 154 results in the generation of electrical osclllations which correspond in wave shape and frequency to the vibrations of the reeds.

The ungrounded input 152, and hence the electrical osc llations generated by the reeds are connected through an input capacitor 168 to a resistor 170 paralleled by a potentiometer 1.72, the parallel combination being grounded as at 1174. A sliding tap 176 on the potentiometer is connected to the control grid 178 of an amplifier tube 180, conveniently onehalf of a l2AU7. The cathode 182 of the tube is grounded through a cathode load resistor 184. The plate 186 is directly connected to the control grid 188 of a phase inverter tube 190, conveniently the other half of the aforementioned 12AU7 tube.

The cathode 192 of the tube 1% is grounded through a cathode load resistor 194, and the plate 196 is connected through a plate load resistor 19-8 equal in value to the cathode load resistor 194 to the B+ bus line 148 through the decoupling filters 158,164, and 160, 166. A capacitor 200 is connected from the phase inverter grid 188 to ground, as is a series combination of a resistor 202 and a capacitor 204. The grid also is connected through a resistor 206 to the B+ bus line through the two de coupling filters just mentioned. 3

The plate output from the phase inverter tube is connected through a capacitor 208 to the grid 210' of an amplifier tube 212, conveniently one half of another 12AU7- tube. "The grid of this tube is grounded through a grid resistor 214, and the cathode 216 is grounded through a cathode resistor 218. The cathode output is similarly connected through a capacitor 220 to the grid 222 of a parallel amplifier tube 223, conveniently the other half of the second 12AU7 tube. The grid similarly is grounded through a grid resistor 224, and the cathode 2216 is grounded through a resistor 228.

The plate 230 is connected through a plate load resistor 2 32 and through the decoupling filter 160, 166 to the B+ bus line. The plate 234 of the amplifier tube 223' is similarly connected through a load resistor 236 and the decoupling filter 160, 166 to the B+ bus line.

The plates 230 and 234 are respectively connected through capacitors 238 and 240 to the control grids 242 and 244 of matched power output tubes 246 and 248 which conveniently may be of the 6AQ5 type. The cathodes are connected in parallel to a common cathode biasing circuit comprising a resistor 250 paralleled by a capacitor 252 and grounded at 254. The control grids 242 and 244 are respectively connected through grid biasing resistors 256 and 258 to ground.

The screen grids of the two power output tubes are connected in parallel to the B-]- bus line, and the suppressors are connected to their respective cathodes in accordance with conventional practice. The plates are connected to the opposite ends of an output transformer 260 having a center tap 262 connected to the B+ bus line.

The output coil of the output transformer 260 is connected in parallel to the jack 96 and to the voice coil 262 of the loudspeaker 74. The connection to the loudspeaker is made through a wire 264, a pair of contacts 266 in the jack 94, a Wire 268, and a wire 270. The jack 94- is arranged so that upon insertion of a plug the contacts 266 will be opened to remove the voice coil 262 from the circuit, and complementary contacts 272 will be closed to connect a resistor 274 in place of the voice coil and in parallel with the ear phones. This resistor is a 7.5 ohm resistor and causes a substantially constant impedance to be presented to the output of the amplifier, it being understood that the very high impedance of ear phones relative to the impedance of the speaker voice coil would otherwise materially change the impedance presented to the output of the amplifier.

The amplifier as just described is capable of a broad band response, being substantially flat from 40 c.p.s. to 20,000 c.p.s A capacitor 276 which in an exemplary embodiment is .02 microfarad is connected across the plates of the power output tubes 244 and 246, thereby materially cutting the treble response. The bass response as noted heretofore is cut by the spacing of the speaker from the case, by the small area of the speaker slots, and by the low quality of the speaker. Thus, both electrical and mechanical features are availed of to render the tonal output similar to that of a piano. The broad band amplifier aside from the output thereof is necessary properly to reproduce all of the partials and properly to reproduce the steep wave fronts produced when any of the reeds is struck by its associated hammer. The necessity of the broad band amplifier will be more apparent when it is considered that a Fourier series corresponding to a steep front wave such as a square wave contains a great many factors multiplied many times in frequency over the fundamental frequency.

The loading of the speaker, and hence the characteristics of the tonal output, further is controlled by the ports in the ends of the case which help to determine the back loading of the speaker cone. The electrical nature of the piano,

and the physical arrangement thereof including the disposition of the various parts all cooperate to make the piano small and light in weight, and hence readily portable.

The specific structure herein shown and described is to be understood as being exemplary only, and is not to be construed as being limitative of the invention. Various changes in structure will doubtless occur to those skilled in the art, and will be understood as forming a part of my invention insofar as they fall within the spirit and scope of the appended claims.

I claim:

1. An electronic piano comprising a substantially completely closed case of generally wide and shallow construction and having an aperture therein, said case having a top and bottom, a plurality of piano actions mounted in said case on the bottom thereof and including a plurality of piano keys opening forwardly of the case and a plurality of hammers respectively operatively connected thereto, a plurality of vibratory reed tone generators mounted in said case and percussively actuated by the hammers of the piano actions, electronic amplifying means within said case for amplifying the electrical oscillations generated by said reeds when vibrating, said amplifying means having an essentially flat frequency response substantially throughout the audio range, a loudspeaker, means for mounting said loudspeaker in said case facing said aperture in said case and spaced from the adjacent portions of the case and providing an air bypass, said loudspeaker being inherently deficient in bass response whereby to limit the bass oscillations as transduced by the loudspeaker, said air bypass further limiting the bass response, a capacitor connected across the output of said amplifying means to limit the high frequency output thereof, and restricted back-loading port means in said case displaced from said speaker.

2. An electronic piano as set forth in claim 1, wherein the aperture is in the top of the case, and wherein the loudspeaker depends from said top and opens up through said aperture.

3. An electronic piano as set forth in claim 1, wherein the case further includes a pair of opposite end walls, wherein the restricted back-loading port means comprises a pair of horizontally elongated slots respectively in said end walls and adapted to act as hand grips for lifting said piano.

4. An electronic piano as set forth in claim 1, wherein the piano actions are mounted on the bottom of said case.

References Cited in the file of this patent UNITED STATES PATENTS D. 154,751 Cushing et al. Aug. 9, 1949 579,031 Zintzsch Mar. 16, 1897 2,001,722 Hammond May 21, 1935 2,089,165 Oliver Aug. 3, 1937 2,187,251 Severy Jan. 16, 1940 2,198,243 Bussard Apr. 23, 1940 2,200,718 Miessner May 14, 1940 2,413,062 Miessner Dec. 24, 1946 2,480,131 Hammond Aug. 30, 1949 2,481,186 Zuch Sept. 6, 1949 2,489,653 Leslie Nov. 29, 1949 2,694,463 Robbins et al. Nov. 16, 1954 2,799,196 Alvarez July 6, 1957 FOREIGN PATENTS 454,069 Great Britain Sept. 23, 1936 

